1. Field of the Invention
This invention relates to a data communication apparatus which performs data communication according to communication procedures.
2. Description of the Related Art
For example, a facsimile apparatus has been known as an apparatus of this kind.
Conventional standard facsimile communication procedures comprise, as provided in CCITT (Comite Consulatif International Telegraphique and Telephonique) recommendation T.30, communication of a low-speed (300 bps (bits per second)) communication control code portion and communication of a high-speed (2400-9600 bps, or higher if possible) image data portion. FIG. 2 illustrates communication procedures of T.30.
In communication using communication procedures provided by CCITT recommendation T.30, communication time is determined by the amount of data of the image data portion.
Even if the amount of image data is reduced using an encoding method having a high compression ratio, such as an MR method or the like, there is a limitation in data compression. Therefore, there is also a limitation in the reduction of the time required for communication. The situation is the same for communication procedures in a polling operation shown in FIG. 12.
In order to overcome such problems, the assignee of the present application has proposed, in U.S. patent application Ser. No. 07/857,313 (filed Mar. 25, 1992), a technique for reducing the time required for communication procedures. In this technique, a calling side receives 300-bps NSF (nonstandard facilities), CSI (called subscriber identifcation) and DIS (digital identification signal) from a called side. When it has become clear that the called side has particular communication procedures, the calling side transmits 300-bps NSS (nonstandard facilities set up) to the called side, and then performs communication of a communication-procedure signal at the same high transmission speed as that of an image signal.
In the above-described technique, however, since the calling side shifts to the high-speed communication procedures after receiving low-speed communication procedures from the called side, the time required for communication procedures is not sufficiently reduced.